Terminal deoxynucleotidyl transferase (TdT) is a marker for immature T lymphocytes. Its presence in SLE or RA peripheral blood lymphocytes (PBL) would support the hypothesis that defective T cell maturation is involved in the pathogenesis of these diseases. On the other hand, its absence in PBL would support the hypothesis that T cells are differential in response to putative antigens. The relationship between toxoplasmosis and the pathogenesis of polymyositis will be explored via studies of humoral and cellular immunity to Toxoplasma gondii. Seven patients with myositis and serologic evidence of recent toxoplasmodis have been studied in assays of cellular immunity. In general, reactivity of peripheral blood lymphocytes to three mitogens (phytohemagglutinin, concanavalin A, and pokeweed mitogen) were good, that is 50% of expected or better, whereas reactivity to toxoplasma antigen was markedly less. Because of the female predominance of systemic lupus erythematosus (SLE) and the immunologic role of sex hormones, 16 male subjects fulfilling the ARA criteria for SLE were studied to assess genetic and hormonal status. While chromosomal abnormalities were absent and clinical indicators of altered androgen status were few, the finding of hyperestrogenemia and hypoandrogenemia in some male lupus patients supports the hypothesis that female sex hormones may create an immunologic milieu that facilitates autoimmune phenomena.